1. Field of the Invention
The invention relates to a method of producing hybrid seeds of crops, and more particularly to a method of producing hybrid seeds for annual cotton by cultivating perennially.
2. Description of the Related Art
Although the advantages of cotton hybridization is significant, almost all existing hybrid seeds are planted in main cotton production areas by artificial emasculation hybridization using annual upland cotton (Gossypium hirsutum) or annual Gossypium barbadense parent strain, or by hybridization of an annual male sterile line and a restorer line. The main disadvantages are summarized below:                1) land must be ploughed up annually, and parent strain seeds must be sowed and bred annually.        2) roguing for parental propagation and hybrid seed production must be carried out annually.        3) although it is easy to find a restorer line and form heterosis crosses by using a genic male sterile line, the sterile line is difficult to propagate. This could be achieved generally by hybridization of male sterile plants (genotype: msms) and fertile sister plants thereof with fertile hybrid gene (genotype: Msms) which are isolated from a dual-use genic male sterile line, and the seed collected from the sterile plants still represents 1:1 in male sterile plants and male fertile plants for propagation. Therefore, 50% of the male fertile plants in female parent area must be removed from the hybrid seed production field each year, which is a concentrated and labor-intensive work. As cotton has an indefinite inflorescence with long flowering period, once the temperature exceeds 35° C., some male sterile strains will produce small or micro amount of pollen, which will affect the purity of hybrid seeds. In addition, because of non-uniform distribution of fertile plants and sterile plants, often fertile plants have to be removed continuously. It not only causes a waste of land, but also increases seed production cost.        (4) Although backcrossing of a cytoplasmic male sterile line and maintainer line can maintain male sterility, most of the existing cytoplasmic male sterile lines has harknessii cytoplasm, in which the restorer line is less and the hybridization generation contains adverse effects of sterile cytoplasm. Other types of cytoplasmic male sterile lines are prone to have genetic drift due to years of generative propagation sexually, leading to occurrence of fertile plants in a sterile line.        
In spite of the fact that artificial emasculation hybridization needs high investment and cost, since there is no other better methods for hybrid seed production, artificial emasculation hybridization is still a main method for hybrid seed production. To reduce the seed production costs, generation F2 is generally used in cotton production.